The present invention relates to apparatus for coupling electrical energy into and/or out of a surface acoustic wave ("SAW") device. More particularly, the present invention relates to apparatus for coupling energy into and out of a SAW device which serves as a transponder in a "passive interrogator label system".
A "passive interrogator label system", so-called, is a radar system utilizing transponders which are capable of receiving an interrogating first signal, processing this signal and transmitting a second signal, in reply, that is derived from the first signal and contains encoded information. Because the encoded information normally includes an identification code that is unique to each transponder, and because the transponders of such a system are relatively light weight and small and may be easily attached to other objects to be identified, these transponders are sometimes referred to as "labels". Furthermore, the transponders, which may be implemented by SAW devices, carry no self-contained power source, such as a battery, that must be periodically replaced. Consequently, these transponders are denominated as being "passive"; hence the name "passive interrogator label system".
Passive interrogator label systems of the type to which the present invention relates are disclosed, for example, in the following U.S. patents:
U.S. Pat. No. 4,737,789 of Paul A. Nysen for "Inductive Antenna Coupling for a Surface Acoustic Wave Transponder";
U.S. Pat. No. 4,703,327 of Anthony J. Rossetti and Paul A. Nysen for "Interrogator/Receiver System for Use With a Remote Transponder"; and
U.S. Pat. No. 4,737,790 of Halvor Skeie and Donald Armstrong for "Passive Interrogator Label System with a Surface Acoustic Wave Transponder Operating at its Third Harmonic and Having Increased Bandwidth".
In general, a passive interrogator label system includes an "interrogator" for transmitting a first radio frequency signal; at least one passive transponder which receives this first signal, processes it, and sends back a second radio frequency signal containing encoded information; and a receiver, normally located next to the interrogator, for receiving the second signal and decoding the transponder encoded information.
The passive transponder, as disclosed in these patents, comprises a SAW device and a dipole antenna, either electrically or inductively coupled to the SAW transducers on the SAW device. The dipole antenna is formed by printing conductive ink (e.g. silver) on a substrate in the prescribed antenna pattern, or by stamping a metal foil into the size and shape of the antenna pattern and bonding this foil by heat and pressure to the substrate (e.g. a polyethyline coated Mylar sheet). The antenna with its substrate are therefore relatively flat and, depending upon the radio frequency of operation, is relatively modest in size.
For example, the antenna characteristics required for this application, operating at a frequency of approximately 915 MHz., determine that the antenna be physically approximately one inch wide and three inches long. The SAW device, to which the antenna is coupled, is also placed on the substrate to form the complete assembly or "label".
This flat, card-like structure is ideal for many applications such as identification badges for personnel and ID tags for automobiles, factory parts or the like.
The aforementioned U.S. Pat. No. 4,737,789 describes means for inductively coupling a SAW transponder to an antenna by means of an inductive loop formed by at least one turn of an electrical conductor and having an inductance which is substantially equal to the capacitance presented at the terminals of the SAW device within the frequency range of operation. The two free ends of the loop are electrically connected to the two transducer terminals of the SAW device so that the loop and the device form a resonant circuit at the frequency range of operation.
The U.S. Pat. No. 4,737,789 also describes a package comprising a substrate formed by a Mylar sheet coated on both sides with a polyethyline based, thermally activated adhesive; a SAW device comprising a lithium niobate piezoelectric substrate and a partial turn of a metal foil inductive loop, bonded to the Mylar substrate and electrically connected to the two terminals of the SAW device. This entire assembly is then encapsulated and placed next to an inductive loop which is electrically connected to, or formed integrally with, the antenna.
While the SAW device and its associated inductive loop, so connected, may be conveniently packaged together in a glass or ceramic envelope to form a separate manufactured unit, which may be then mounted adjacent an antenna structure, this package assembly necessarily requires several manufacturing steps to produce. This package is also relatively thick in its final form. Whereas the physical size of the assembly is not significant in many applications, certain applications, such as wallet size ID tags or cards, require the assembly to be as thin as possible.
The commonly-owned U.S. Pat. Application No. 07/434,232 of Paul A. Nysen, Jon L. Nagel, and Richard D. Smith entitled "Inductively Coupled SAW Device and Method of Making the Same", filed concurrently herewith, discloses a SAW device having a conductive loop deposited on the active surface of the piezoelectric substrate which is electrically connected at its ends to the two transducer terminals of the device. The conductive loop forms an inductive element for coupling energy into and out of the SAW device. The active surface containing the acoustic paths of the SAW device is protected by a nonconductive cover disposed above, and attached to, the surface by means of an adhesive. This cover may be made of glass, for example.
This device, comprising the SAW substrate with the inductive loop pattern and its associated cover is preferably placed in a protective frame which laterally surrounds the SAW substrate and cover. The protective frame, which may be made of metal to provide resilience and strength, may have the same thickness as the combined thickness of the substrate and cover. This entire package, including the substrate, cover and frame is therefore extremely small in its dimensions and yet mechanically strong and completely passivated and protected against outside elements.
The reduction in size of the SAW device package brought about by placing the inductive loop on the SAW substrate itself creates a problem of alignment and registration with the corresponding inductive loop associated with the antenna. Significant loss of signal results when the SAW device, with its inductive loop, is not precisely aligned and packaged with the inductive loop of the antenna.